Wireless power transfer (WPT) based on the magnetic resonance and near-field coupling of two loop resonators was reported by Nicola Tesla a century ago. It transmits electric energy and information from a transmitter to the receiver without physical contact. Since the 1960s, and particularly over the last few decades, WPT has been an active research area in regard to transcutaneous energy systems for medical implants. For modern short-range applications, inductive power transfer (IPT) systems and wireless charging systems for portable electronic equipment have attracted much attention since 1990's and 2000's respectively. Wireless charging technology for portable electronic devices has reached commercialization stage through the launch of the “Qi” Standard by the Wireless Power Consortium, now comprising over 135 companies worldwide. The recent research activities on this topic have focused on mid-range applications comprising 2-coil, 4-coil, relay resonators and domino-resonator systems.
At present, a lot of research is focused on improving the performance of WPT systems, such as to increase the transfer distance, to improve the efficiency and to widen the operating frequency. There is a lack of research examining and addressing the problems arising from the receiver side, which also has a significant influence on the performance and efficiency of the system. In most of the applications, the AC power output from the receiver is first converted to a DC voltage. The simplest approach is to use a diode rectifier 10 with a capacitor 12 connected at the DC output side as shown in FIG. 1(a). However, this capacitor is charged to a value close to the peak of the AC input voltage (see FIG. 1(b)). As a result, the input current is very large near the peak of the AC input voltage and it does not flow continuously. These diode rectifiers draw highly distorted current from the AC source and result in a poor power factor (PF). The energy efficiency and power transfer capability of a poor PF system are relatively low because of the high conduction loss in the power converters and transmission wires. Additionally, the distorted current has rich harmonic content which may create extra electromagnetic interference (EMI) in neighboring electronic equipment.
An electronic power converter, such as a boost converter, can be used to shape the line current drawn by the rectifier so it is sinusoidal and in phase with the line voltage. FIG. 2(a) shows a classical boost converter connected after a diode bridge rectifier to form a power factor correction (PFC) circuit. The output dc voltage Vdc actual is sensed and fed to an error amplifier 20. The difference Verror between the actual and reference voltage Vdc ref is derived and applied to a compensator circuit 22, such as a proportional-integral (PI) compensator. The output of the compensator is multiplied in circuit 24 with the signal proportional to the line voltage waveform νS to produce the reference current signal iL,ref. Afterward, a current-mode controller 26 is used to generate the on and off signal to the switch 28 shaping the current waveform of the inductor 11. Therefore, the average waveshape of the line current is forced to follow the waveform of the line voltage.
FIG. 2(b) depicts the current waveform of the converter. It can be observed that the switching frequency of the PFC converter must be many times higher than the frequency of the AC system. The typical operating frequency of a modern PFC converter is in the range of 20 to 40 kHz which is over 100 times higher than the frequency of the AC system. Using a 400 kHz WPT system as an example, applying this current-shaping technology implies that the switch 28 has to operate at 40 MHz. In fact, when the switching frequency approaches tens of megahertz, the switching loss becomes significance and the efficiency of the converter is sharply reduced. The converter also has a number of other problems and limitations, including parasitic inductance and capacitance of the interconnections, stray inductances of the magnetic components, and parasitic and junction capacitances of the semiconductor diodes and switches. The EMI, thermal, insulation and isolation problems of the converter are not easy to solve individually. Hence, the conventional current-shaping PFC technology is not suitable for high frequency ac system since the fundamental frequency is already in the range of hundreds of kilohertz to a few megahertz.